The present invention relates to a process for economically and safely producing difluoromethane, which comprises fluorinating dichloromethane with hydrogen fluoride in a liquid phase in the presence of a catalyst.
Difluoromethane (hereinafter referred to as xe2x80x9cR-32xe2x80x9d) has become a center of attention as a substitute refrigeration medium for chlorodifluoromethane which is conventionally used as a refrigeration medium for room air-conditioner.
It is known that R-32 is produced by reacting dichloromethane (hereinafter referred to as xe2x80x9cR-30xe2x80x9d) with hydrogen fluoride (hereinafter referred to as xe2x80x9cHFxe2x80x9d) in a gas or liquid phase in the presence of a catalyst.
U.S. Pat. Nos. 2,749,374 and 2,749,375 disclose a process wherein R-30 is reacted with HF in a liquid phase at a temperature within the range from 110 to 175xc2x0 C. in the presence of an antimony chloride fluoride catalyst (SbClxFy, x+y=3 to 5, y/(x+y) greater than 0.8, Sb(V) greater than 5%) to obtain R-32. In this process, however, a large amount of R-40 series compounds such as monochloromethane (hereinafter referred to as xe2x80x9cR-40xe2x80x9d) and fluoromethane (hereinafter referred to as xe2x80x9cR-41xe2x80x9d), which are undesired impurities other than R-30 series compounds (R-32, R-31 and R-30), are formed as by-products. It is known that HF and antimony halide corrode the reaction apparatus and it is extremely important for the production of difluoromethane that the reaction system does not corrode the reaction apparatus. However, the above patent does not disclose that the material of a reactor shows a corrosion resistance in case of reaction under the above conditions.
Japanese Patent Kokai Publication No. 1-36556 discloses a method of adding antimony trihalide to antimony pentahalide as a method for prevention of corrosion of a vessel which is normally corroded by a mixture of HF and antimony pentahalide.
In the above Publication, there is a description that the reaction temperature is not specifically limited. However, the reaction temperature is not more than 50xc2x0 C. in all Examples and Comparative Examples and a corrosion preventing effect capable of enduring practical application at more than 80xc2x0 C. can not be assumed. Also, there is no description of the corrosion preventing effect for Hastelloy. There is also described that xe2x80x9cthis reaction is generally applied to a trihalomethyl compound, and is also applied to a 1,1-dihalovinyl compound which is a precursor of said compoundxe2x80x9d and the Examples do not disclose a reaction example using R-30.
Japanese Patent Kokai Publication No 59-231030 discloses a process wherein R-30 is reacted with HF in a gas phase at a reaction temperature within the range from 200 to 500xc2x0 C. in the presence of an aluminum fluoride or chromium fluoride catalyst to obtain R-32. However, it can not be said that this process is an economical process from industrial points of view because the reaction temperature is high (200-500xc2x0 C.) and a complicated apparatus for recovering/recycling the unreacted HF and R-30 is required.
An object of the present invention is to solve these problems in conventional techniques, thereby providing a process of economically and safely producing R-32.
An aspect of present invention resides in a process for producing difluoromethane comprising reacting dichloromethane with hydrogen fluoride in a liquid phase in the presence of a fluorinating catalyst, wherein the reaction is conducted at a temperature within the range from 80 to 150xc2x0 C. under an absolute pressure within the range from 8 to 80 kg/cm2 using a mixture of antimony pentafluoride and antimony trifluoride, or antimony pentafluoride having a concentration of not more than 2% by mol based on hydrogen fluoride in a liquid phase mixture, as the fluorinating catalyst.
Although the cost is most cheap when R-30 is used as a starting material in the process of the present invention, chlorofluoromethane (hereinafter referred to as xe2x80x9cR-31xe2x80x9d) can also be used. R-31 is an intermediate of the reaction wherein R-30 is reacted with HF to obtain R-32 and, therefore, only R-30 or a mixture of R-30 and R-31 may be used as the raw material. Since fluorination of R-30 and R-31 proceeds successively, it can be conducted as a series of reactions.
In the process of the present invention, a mixture of antimony pentafluoride and antimony trifluoride is used or antimony pentafluoride alone is used as the fluorinating catalyst.
When an amount of antimony trifluoride in case of using the mixture of antimony pentafluoride and antimony trifluoride as the fluorinating catalyst is decreased, the corrosion of the reactor is liable to proceed. When the amount of antimony trifluoride is increased, the reactivity is lowered. Accordingly, a molar ratio of antimony pentafluoride to antimony trifluoride, which is easily used, is normally within the range from 1:1 to 1:5. In this case, the mixture of antimony pentafluoride and antimony trifluoride in the reaction system is normally used in an amount within the range from 0.2 to 10% by mol, preferably from 2 to 8% by mol, based on HF in the liquid phase mixture. When the amount of the fluorinating agent exceeds 10% by mol, the reaction is not influenced but corrosion of the reactor becomes severe. On the other hand, when the amount is smaller than 0.2% by mol, the reaction proceeds but the formation rate of R-32 is small and the productivity per volume of the reactor is lowered.
When using only antimony pentafluoride as the fluorinating catalyst, antimony pentafluoride is normally used in an amount within the range from 0.1 to 2% by mol, preferably from 0.2 to 1% by mol, based on HF in the liquid phase mixture. When the amount is larger than 2% by mol, the corrosion of the reactor becomes severe. On the other hand, when the amount is smaller than 0.1% by mol, the formation rate of R-32 becomes small.
Antimony pentafluoride and antimony trifluoride used in the process of the present invention can be formed in situ, by fluorinating antimony chloride with a sufficient amount of HF. It is also possible to prepare antimony pentafluoride by charging antimony trifluoride or antimony trichloride, followed by chlorinating with chlorine and then fluorinating.
In the process of the present invention, HF and R-30 in a liquid or gas state are fed into a mixed liquid of HF and a fluorinating catalyst, wherein the concentration of the fluorinating catalyst has been controlled within the above range, and then R-30 is fluorinated in the liquid phase. Although the molar ratio of HF to R-30 to be fed is normally about 2:1 (stoichiometric amount), it is necessary to change the molar ratio according to the composition of the drawing gas and the composition of the recycling gas in case of recycling. It is preferred that HF is present in the liquid phase mixture in an amount of at least 5 mol per one mol of R-30. It is also possible to feed only R-30 if this value can be maintained. It is considered that a part of antimony as the catalyst is chlorinated under such a reaction condition.
The reaction temperature is preferably within the range from 80 to 150xc2x0 C., more preferably from 90 to 120xc2x0 C. The reaction proceeds even if the reaction temperature is lower than 80xc2x0 C., but it is not suitable for practical application because the formation rate of R-32 is low and the productivity per volume of the reactor is poor. On the other hand, when the reaction temperature exceeds 150xc2x0 C., the corrosion rate of the reactor is increased.
The reaction sufficiently proceeds if the charging rate to the catalyst is, for example, up to about 5 (R-30 mol/Cat. mol)/hour at 100xc2x0 C.
In the process of the present invention, it is necessary to maintain the pressure conditions of the gas phase so that HF can be present in the liquid state at the above reaction temperature. For example, it is necessary to adjust the pressure to an absolute pressure of at least about 6.6 kg/cm2 at 80xc2x0 C. It is preferred to use the condition wherein HCl and R-32 (the reaction product) can be removed by distillation without drawing HF and R-30 (the reaction raw material). Since HCl boils at xe2x88x9285xc2x0 C. and R-32 boils at xe2x88x9252xc2x0 C. under the pressure of 1 atm, the reaction pressure becomes higher so that the distillation can be conducted at the more advantageous temperature, i.e. higher temperature. However, it increases the plant cost to maintain the high pressure. Accordingly, the process of the present invention is preferably conducted under the pressure of the gas phase which is an absolute pressure within the range from 8 to 60 kg/cm2, more preferably from 10 to 50 kg/cm2.
The process of the present invention can be conducted using a conventional apparatus which is generally known. The apparatus may be an apparatus comprising a single reactor, capable of feeding the starting material (R-30 and HF) in the liquid or gas state to the reactor and capable of heating or cooling enough to constantly maintain the reaction temperature. It is necessary that the reactor promotes contact between the reaction substances by a proper mixing method and endures the reaction pressure. It is preferred to make it possible to draw a part of the reaction mixture from a reflux condenser by providing this reactor with a reflux column and the reflux condenser. Therefore, it becomes possible to avoid entrainment of the catalyst into an effluent gas flow from the reactor and to remove HCl and R-32 as the product having comparatively low boiling point by distillation while unreacted HF and R-30 which are a substance having comparatively high boiling point and R-31 which is the intermediate product in this gas flow are remained. It is more preferred to connect the reflux column directly to the upper part of the reactor.
The material of the reactor must have a corrosion resistance enough to endure practical application under the conditions of the present invention. For example, there can be used Inconel 600, NAR25-50MTI, Hastelloy C, Hastelloy G-30, double-phase stainless steel, Hastelloy C-22 and the like. It is particularly preferred to use Hastelloy C.
The reaction mixture contains R-31 as the intermediate product and unreacted R-30 and HF, in addition to R-32 and HCl as the reaction product. These can be separated by a usual fractionation process and R-32 as the desired substance can be obtained. R-31, R-30 and HF may be used with recycling. When the unreacted substance is recycled, the molar ratio of the raw materials (HF/R-30) to be newly fed may be about 2 which is closer to a stoichiometric value. When only R-32 and HCl as the reaction product are drawn from the reflux condenser as described above, the separation of the desired substance R-32 becomes easier.
According to the preferred embodiment of the present invention, the process of the present invention is conducted by the following steps:
(1) First, a fluorinating catalyst and HF are changed in a reactor.
(2) A mixture of HF and R-30, or R-30 is added to react them. In the liquid phase mixture, HF is used in an amount that the concentration of the fluorinating catalyst is within the above range. The reaction is conducted under the above conditions to form R-32, HCl and R-31 as an intermediate product.
(3) A part or all of the reaction mixture is drawn.
(4) The drawn reaction mixture is separated by a distillation to give the desired R-32.
(5) Unreacted HF, R-30 and R-31 are optionally returned to the reactor.
The above process can be conducted by a batch process but is preferably conducted by a continuous process.
The following Examples and Comparative Examples further illustrate the present invention in detail.